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Chapter 5Upstream channel capacity and characterisation pcell for the ‘Contention Access Message Length’ parameter (the other 1% was queued inthe station’s buffer). From this 36%, only 14.5% (Figure 5.7) of the offered load wastransmitted in contention access, and the rest (21.5%) was sent using reservation access.In order to transmit the 1-ATM-cell messages and the reservation requests of the othermessages using contention access, Figure 5.8 reveals that 10.8% of the bandwidth wasconsumed by collisions. With a value of 5-ATM cells for the same parameter, thebandwidth consumed by collisions was reported in the order of 16% of the cc, whichresults in a loss in system throughput as seen in Figure 5.6.Thus, it can be noticed that as larger messages are allowed to be transmitted usingcontention access, the risk of collision is increased slightly. In the case that none of themessages were transmitted in contention access, it is obvious that with heavy trafficloads, the bandwidth consumed by collisions is minimised.5.4.4 Effects of reservation request sizeIn this analysis, we now address the case when stations only use reservation access forthe transmission of data messages. The analysis focuses on the impact on systemperformance when the maximum request size (or ‘Maximum Reservation AccessMessage Length’ as named in the specification) is ranged from 6 to 32 ATM cells. Thisconfiguration parameter specifies the maximum length of a message that can betransmitted using a single reservation request access. Any message greater than this istransmitted by making multiple reservation requests.When a station is enabled to use only reservation access for the transmission of datamessages, as the offered load increases, short packets tend to accumulate in the station’sbuffer. This is true even at lower loads depending on the traffic type. Large packets forsome applications may generate several (or many) ATM cells at once when segmentedinto 53-bytes chunks. If a station with a large packet is allowed to send only one requestfor the transmission of the complete packet, that may affect the transmissions of theother stations. This is because the headend serves the station’s reservation requests in aFIFO order regardless of the traffic type, and the larger the request size the higher thechannel access will be, which delays the other reservation request. This effect will beaddressed at the end of this section.5-18